With recent great progress in the electrical and electronic industries, adhesion of electronic parts has become important. Also, thermally conductive adhesives are prepared by thermally dispersing conductive inorganic particles in a polymeric resin in order to control the heat generated by electronic parts. Thermally conductive adhesives function to transfer the heat generated from electronic parts to a heat sink for discharging heat, and to bond electronics parts.
Typically, thermally conductive adhesives for use in removing the heat generated from electronic parts are prepared by adding a thermally conductive filler, such as metal, ceramic, etc., in a resin, such as acrylate, polyurethane or silicone. Recently, higher thermal conductivity has been required in thermally conductive adhesives. Thus, various methods for improving thermal conductivity are being taken, including, for example, development of highly thermally conductive fillers, combination of fillers different in morphology and properties, use of fabric fillers such as non-woven fabric for providing a path for heat conduction, rearrangement of fillers in the presence of a magnetic field, etc. Improvement of thermal conductivity through the modification of structures of thermal conductivity fillers, however, entails a difficulty in hardness control of adhesives and an increase in production cost as well as the necessity of improving the bonding area at irregular/projecting areas. Separate from the development or structure modification of thermally conductive fillers, increasing the bonding area by improving the properties of resin itself is also being studied intensively in order to improve the efficiency of heat transfer.
If there are irregular loci between electronic heating parts and thermally conductive compositions or between heat dissipating members and thermally conductive composition, a visible air layer which is concerned with bonding area having significantly bad influence on heat transfer performance is found therebetween.
To overcome this obstacle, various methods have been suggested, including softening heat dissipating pads to reduce their hardness, embossing the surface of the adhesive sheet used, softening the adhesive sheet, etc.
One of the methods of softening adhesive sheets is to take advantage of foams so as to reduce their hardness. Because they reduce noise and vibration as well as soften resin compositions, the introduction of foams has extensively been studied.
WO99/03943 discloses a double-sided adhesive tape having hollow polymeric microspheres dispersed in a crosslinked adhesive sheet. The dispersed hollow polymeric microspheres can endow the adhesive tapes with restorative power upon compression, but increase the hardness of the adhesive sheet, which offsets the reduction of hardness due to the foam.
Japanese Pat. Laid-Open Publication No. 63-225684 describes a method of simultaneously introducing a crosslinking structure and a foam structure into an acrylic polymer by using UV light radiation in the presence of a diazonium salt compound. However, this method requires that the materials be designed not to interrupt the permeation of UV light. Also, it is difficult to control the size and distribution of foam cells.
A thermal treatment method, which avoids the problem of UV permeation, is disclosed in Japanese Pat. Laid-Open Publication No. 55-90525 which describes a pressure-sensitive adhesive foaming composition, which is thermally crosslinked and foamed. The compounds used for this method are poor in cohesion therebetween and in processability because they have low molecular weights. This method also has difficulty in controlling the size and distribution of foam cells, so that the adhesive composition does not show consistent properties whenever it is prepared.
In order to overcome the problems entailed by the use of low molecular weight compounds, Japanese Pat. Laid-Open Publication No. 2002-80817 suggests thermal curing and foaming treatment of acrylic copolymers having a high molecular weight of 100,000 or more to prepare crosslinked foamed adhesives. The simultaneous execution of thermal curing and thermal foaming according to the prior art cause the size and distribution of foam cells to vary with temperature so as to result in poor uniformity of the properties of the final product. Particularly, the adhesive sheet has non-homogeneous surfaces, which reduce the expansion of the bonding area.
Korean Pat. Laid-Open Publication No. 2003-0092759 describes a UV-cuing foaming resin composition comprising photopolymerizable urethane acrylate oligomers, acetal-based compounds, a photo-decomposable foaming agent, such as sulonium salts or azo-based compounds, and a photocatalyst. Although undergoing chemical foaming in the presence of UV light, it is very difficult to control the size and distribution of foam cells and to set working conditions. The UV photolysis of the photoinitiator produces too many radicals, which makes the resin a low molecular weight structure, deteriorating cohesive properties of the adhesive. In addition, when the products of this method are used for a long period of time at high temperatures, the nitrogen gas, which is trapped in the resulting foam cells, may leak, exerting a bad influence on the long-term reliability of the adhesive.
Korean Pat. Laid-Open Publication No. 1988-0002964 discloses a foamed pressure-sensitive adhesive sheet with thermoplastic hollow polymeric microspheres having a very low density dispersed therein. The hollow microspheres in the adhesive sheet hinder the increase in the bonding area of the adhesive sheet with regard to glass or metal substrates due to their elasticity. Further, when used for a long period of time at high temperatures, the microspheres expand, which separates the adhesive from the substrate.